Improved output window structure for microwave tubes

ABSTRACT

High-frequency tubes, such as klystrons or travelling wave tubes for example, producing high-power outputs within wide frequency bands by means of output devices comprising a dielectric window which ensures a seal between the evacuated enclosure of the tube and the load circuits which take the form of pressurized waveguides. This window is arranged in a waveguide section whose advantageously circular cross-section is larger than that of the output waveguide of the tube and that of the load waveguides, the connections between the waveguides being effected through junctions. The invention provides means which ensure to the thus constituted output device a resonance frequency equal to the centre frequency of the operating band of the tube, and means which effect impedance-matching throughout this band.

United States Patent [1 1 Firmain et al.

[ Nov. 27, 1973 IMPROVED OUTPUT WINDOW STRUCTURE FOR MICROWAVE TUBES[75] Inventors: Gerard Firmain; Guy Egloff, both of Paris 8eme, FranceOTHER PUBLICATIONS Lebacqz et al., High Power Windows At MicrowaveFrequencies, lEE Paper No. 2675R 12/1958, pp.

Chen, T. S., Broadbanding of Resonant-Type Microwave Output Windows, RCAReview, 6/1954, pp. 204-207.

Primary Examiner-Rudolph V. Rolinec I Assistant E.raminer-Wm. H. PunterAttorney-John W. Malley et al.

[5 7 ABSTRACT High-frequency tubes, such as klystrons or travelling wavetubes for example, producing high-power outputs within wide frequencybands by means of output devices comprising a dielectric window whichensures a seal between the evacuated enclosure of the tube and the loadcircuits which take the form of pressurized waveguides. This window isarranged in a waveguide section whose advantageously circularcrosssection is larger than that of the output waveguide of the tube andthat of the load waveguides, the connections between the waveguidesbeing effected through junctions. The invention provides means whichensure to the thus constituted output device a resonance frequency equalto the centre frequency of the operating band of the tube, and meanswhich effect impedancematching throughout this band.

6 Claims, 8 Drawing Figures PATENTH] HUVZ 7 [S75 SHEET 1 OF 4 PAIENIEU177K). ('08 SHEET 3 OF 4 MICROWAVE TUBES The present invention relatesto improvements in high-frequency, high-power, wide-band tubes such asklystrons for example or travelling wave tubes, for developing powerswhose peak levels may reach several megawatts and even several tens ofmegawatts.

It relates more particularly to the output elements of such tubes, theseelements conventionally being constituted by wave guides in which adielectric window is provided for the vacuum tight sealing of theenclosure forming the body of the tube.

The design of windows which do not interfere with the transmission ofthe power developed by the tube, throughout the whole of thefrequencyband, is a particularly delicate matter, especially because thedimensions of these windows have to be quite large.

On the one hand, the area presented by the window to the HF wave beingtransmitted must be sufficiently large-to avoid any risk of a shuttereffect due to the electric field prevailing in the window.

On the other hand, the thickness of this window should be adequate toallow it to withstand the pressure difference existing between its twofaces. However, this pressure difference is very substantial since onone side there is the enclosure which is under vacuum and on the otherthere is the waveguide supplying the load circuits, which waveguide isusually pressurised to a level in the order of some few kilograms persquare centimeter to prevent any unwanted ionisation there.

Generally, the tube output is through one, or sometimes two rectangularoutput waveguides whose crosssection is too small for the output windowsto be arranged in them. Since the power delivered by the tube must notbe dispersed amongst parasitic modes, but

.must be confined to the sole fundamental mode, generally the TE in thecase of an output which is effected through one rectangular section ofthe waveguide as described, there can be no question of enlaying thesection of this waveguide sufficiently to be able to install the windowthere if this were done, the cut-off frequency would be too low andparasitic modes would appear.

The solution generally employed consists, therefore, in inserting thewindow in a waveguide portion of larger cross-section than that of theoutput waveguide of the tube, a more or less progressive junction beingprovided to establish the connection between these two waveguides ofdifferent section.

In the usual way, the window is made circular and is inserted in asection of cylindrical waveguide, the junction thus being one of conicalform. This kind of window has two advantages over a rectangular window.On

the one hand, it presents a larger area for the transmit-.

ted electromagnetic wave, whilst having roughly the same size. On theother hand, the parasitic modes in a circular waveguide are further awayfrom one another than in a rectangular section waveguide so that therisk of the appearance of parasitic modes is not so high.

A major problem constituted by output devices of this kind, whether thewindow be circular or not, is that of matching them within the widefrequency band of operation of the tube, this matching beingparticularly difficult owing to the junction. Furthermore, thisdifficulty is generally aggravated by the presence of a second junctionwhich establishes the connection between the output of the waveguidesection containing the window, and a rectangular waveguide known as theload waveguide, the load circuits conventionally comprising rectangularsection waveguides of smaller cross-section than the waveguide whichcontains the window.

A variety of solutions have already been proposed in an attempt toresolve this problem, that is to say in order to give the assembly ofwindow, waveguide section containing same and junctions, a standing waveratio of close to unity within a frequency band which as closely aspossible corresponds with the operating frequency band of the-tube.

One of these solutions consists in utilisinga relatively long circularwaveguide section so that the junctions are sufficiently far away fromthe window to produce a pure real admittance in the plane of its faces.This solution has several drawbacks. It is bulky there is the risk ofstationary waves appearing in the circular wave-- guide section, whichthen behaves as a cavity which is the seat of a free-running oscillationin addition, the impedance-matching has to be carried out in thecircular waveguide section and setting up is thus rendered a delicateoperation.

A second solution consists in utilising more progressive junctions witha very flat slope. This solution has the same drawbacks as before.

A third known solution consists in incorporating a matching obstaclewithin the thickness of the dielectric window itself. Although thissolution enables good matching to be achieved throughout a widefrequency band, the setting up and manufacture of this kind of windoware extremely delicate operations.

One object of the present invention is to produce high-frequency,high-power, wide-band tubes, by the introduction of an improved outputdevice which enables excellent transmission of "the output power fromthe tube throughout its operating frequency range to be achieved, whilstescaping the drawbacks of the devices thus far known and in particularproviding a system which is simple to manufacture and set up.

In an improved tube in accordance with the invention, the matching ofthe output device which comprises at least one dielectric windowarranged in a portion of waveguide whose section is larger than that ofthe rectangular output waveguide section of the tube, and a junctionlinking these two waveguides, is achieved by metallic obstacles arrangedin front of the junction and in the vicinity thereof, within the outputwaveguide itself. A first set of obstacles corrects the effects of thejunction whilst a second advantageously matches the impedance of theoutput device assembly to that of the rectangular output waveguide.

In the case of a second junction located after the window and connectingthe waveguide containing it, with the load waveguide, another set ofmetallic obstacles is arranged after said second junction in the loadwaveguide, symmetrically vis-a-vis those arranged in the outputwaveguide.

Generally, although this is by no means limitative, and for the reasonsalready indicated the output window is circular and thewaveguide-section containing it is cylindrical.

Other objects and features of the invention will become apparent fromthe ensuing description, given by way of non-limitative example andillustrated by the attached figures in which FIG. 1 is a schematicperspective view of a conventional output device for a high-powerhigh-frequency tube FIG. 2 illustrates different resonance curves for awindow, matched or unmatched FIG. 3 illustrates a schematic view of anembodiment of matching means in accordance with the invention FIG. 4 isa schematic view of an output device for a high-frequency tube inaccordance with the invention FIG. 5 is a schematic view of an exampleof klystron tube comprising an output device according to the inventionFIGS. 6, 7 and 8 are schematic views of other embodiments of matchingmeans in accordance with the invention.

FIG. 1 schematically illustrates a conventional output device for ahigh-frequency, high-power wide-band tube, utilising a circulardielectric window.

The tube has not been illustrated here it is symbo lised by an arrowtogether with the word tube" and by its rectangular output waveguide 1.The interior of this waveguide, which forms an integral part of thetube, is under vacuum. The load circuit has not been illustrated here itis symbolised by an arrow together with the word Load. A secondrectangular waveguide 2, forming part of the output device of the tube,enables the tube to be connected to the load circuit in the manneralready described this load waveguide 2 is advantageously pressurizedand carries a gas pressure in the order of some few kilograms per squarecentimeter for example. The waveguides 1 and 2 are linked by a circularwaveguide portion 3 containing a circular vacuumtight window 4 made of astrong dielectric material, ceramic for example, and having a thickness2 and a radius a, the connections between said waveguide portion 3 andthe rectangular waveguides l and 2, being effected through progressivejunctions t of approximatly conical form, generally referred to asconical junctions or tapered junctions.

As already described, the dimensions of the window 4 must besufficiently large. Moreover, they are chosen in such a way that thewindow transmits the TE. mode corresponding, in the circular waveguide3, with the fundamental TE mode which is transmitted by the outputwaveguide 1 so that no parasitic modes appear.

FIG. 2 illustrates the resonance curves of a circular window underdifferent conditions, and enables the object of the invention to be morereadily understood. In this diagram, on the abscissa there is plottedthe frequency F and on the ordinance the standing wave ratio 5. Theoperating frequency band AF of the tube is defined between F, and F Thecontinuous curve 5 is the resonance curve of a circular window such asthat 4 inserted in a circular junction-less waveguide, the dimensions ofthe window being such that it resonates in the TE mode with a standingwave ratio of unity at the centre frequency F of the band AF, theimpedance-matching being assumed to be effected by conventional means.

The dots and dashes curve 6 represents the resonance curve of the samewindow arranged in a circular section waveguide disposed between twoconical junctions connected to rectangular waveguide sections in themanner indicated in FIG. 1.

It is very clear that the presence of junctions produces a substantialshift in the matched frequency of the system. The conventional matchingmeans, which are adequate to produce suitable matching in the absence ofany junctions, in this case no longer allow the system to be matchedwithin the operating frequency band AF of the tube, which band isradically displaced in relation to the resonance frequency F of thesystem.

In accordance with the invention, the output device of a tube comprises,within the rectangular waveguide connected to the waveguide portionwhich contains the dielectric window by a junction 1, metallic obstacleswhich transform the curve 6 of FIG. 2 into a curve 7 centeredon thecenter frequency F of the band AF. The resonance frequency of the windowinserted in the circular waveguide portion and of the junctions, thushaving been made equal to the centre frequency F of the band AF,matching this band is quite readily achievable using conventional meansbrief mention of which will be made at a later point.

FIG. 3 schematically illustrates an embodiment of such metal obstacles,constituted in this case by two metal bars 8 and 9 arranged in therectangular waveguide G near to the junction 1 which connects saidwaveguide G with the circular waveguide portion 3 containing the window(not shown here). These bars are arranged in the same sectional plane ofthe waveguide G, in such a way to be parallel to the electric componentof the electromagnetic wave propagating through said waveguide and to besymmetrically disposed in relation to the waveguide axis, each bar beingconnected to one of the two wider faces of the waveguide they act as aninductive obstacle.

In the case of the tube output device containing only one junctionbetween the output waveguide 1 and the circular waveguide portion 3,said two bars enable the resonance frequency of the window and of theassociated junction to be made equal to the centre frequency F of theband AF of the tube. More generally, the device, as FIG. 4 shows,comprises a second junction between the circular waveguide portion 3 andthe load waveguide 2. In this case, the load waveguide 2 itself containsmetal obstacles in the form of bars 10 and 11 for example, identical tothose in the output wave guide 1 and disposed symmetrically vis-a-visthe window 4. I

Thus, thanks to the presence of these metal obstacles, the design andsetting up of which are extremely simple to put into effect inparticular because they are located in rectangular waveguide sections,the disturbances caused by the junctions are suppressed and theresonance frequency device is made equal to the centre frequency F ofthe operating band of the tube.

As far as the matching of the device throughout the whole of said bandAF, is concerned, this is something which is then simple to carry outand in particular has the advantage of being independent of thecorrection effected by the bars 8 and 9.

9. Two bars and 15 are symmetrically arranged in the load waveguide 2.

An output device of this kind conveniently enables the output power ofthe tube to be transmitted throughout its whole operating frequencyrange, whilst at the same time being quite simple to manufacture and setup. Moreover, since the matching has been achieved without increasingthe length of the system of circular waveguide portion and junctions,the risk of the appearance of free-running oscillation in this system isvery much reduced.

FIG. 5 schematically shows an example of highfrequency tube comprisingan output device according to the invention, the tube thus improvedbeing here a klystron tube.

Such a klystron comprises within a tight exhausted enclosure 80, theclassical elements of a klystron not shown here for clarity of thefigure. The klystron here represented as an example comprises twocavities, an input one 81 provided with an input coupling device 83 andan output one 82 provided with an output device 84 according to theinvention. The emissive cathode is disposed within the lower part of theenclosure 80 and may be heated by means of connections 85, while thecollector electrode is disposed in the upper part 86 of said enclosure.

The output cavity 82 and the output device 84 are shown in a partlysectional view for showing the matching means of the invention. Theoutput device 84 is equivalent to that shown on FIG. 4 and the samereferences correspond to the same elements. Of course two only of thefour metallic rods constituting the matching means in each output 1 andload 2 waveguides are shown here owing to the sectional view. Thedielectric window 4 is represented with dashed lines for the sake ofclarity.

As earlier said, an output device such as 84 may also be used for otherhigh-frequency high-power tubes, traveling wave tubes for example.

Such output devices may also be used for several output tubes, klystronsfor example which may have two output devices such as 84 connected tothe output cavity. I

FIGS. 6, 7 and 8 schematically illustrate other practical embodiments ofmetal obstacles which, in accordance with the invention, enable theeffects of a junction I between a rectangular waveguide G and a circularwaveguide containing a dielectric window (not shown), to be compensated.

In the example of FIG. 6, the two metal bars of FIG. 3 are replaced bytwo flat metal fins 58 and 59 arranged in the same plane of section ofthe rectangular waveguide G, substantially at the same distance from thejunction 1 as the bars 8 and 9 of FIG. 3 these two fins are connected tothe smaller faces of the rectangular waveguide, along a line parallel tothe electric component of the electromagnetic wave propagating there,and act as an inductive window.

In the example in FIG. 7, the inductive window of FIG. 6 is replaced bya capacitive window located in the rectangular waveguide G, at a greaterdistance from the junction, in order to produce the same effect as suchinductive window of FIG. 6. This capacitive window of FIG. 7 isconstituted by two flat metal fins 68 and 69 fixed to the larger facesof the waveguide G in the same plane of section thereof andperpendicularly to the electric component of the electromagnetic wave.

In these two embodiments, FIGS. 6 and 7, the impedance-matching withinthe operating frequency band of the tube, is effected by a techniquewhich is well known per se, through the agency of a second set of metalobstacles, not shown in these figures, but equivalent to the bars 12-and13 of FIG. 4 for example. This second set of obstacles can itself beconstituted rather than by bars such as those 12 and-13, by flat finsconstituting either an inductive'window or a capacitive window andsuitably arranged in the relevant rectangular waveguide.

Another possible embodiment of the invention is that of FIG. 8 where thetwo sets of obstacles, one of which restores the resonance frequency F,FIG. 2) to the centre of the band AF, whilst the other effectsimpedance-matching within this band, are combined, the system beingconstituted by a resonant window 70 arranged in the relevant rectangularwaveguide G. This window can be considered as consisting of theconbination of capacitive obstacles (such as those 68 and 69 of FIG. 7)which bring the frequency F to F (FIG. 2), and inductive obstacles (ofthe kind represented by 58 and 59), which effect impedance-matchingwithin the band AF. In order for the planes containing these two sets ofobstacles to coincide and for the obstacles to combined to constitutethe resonant window 70, it is merely necessary to select the length ofthe circular waveguide portion 3 appropriately. This arrangement,however, has the drawback of being rather more delicate to set up and oflengthening the waveguide portion 3, with the attendant risk of thedevelopment of parasitic modes.

As already indicated, --the description isconcerned more particularly,albeit not exclusively, with a circular window tube output device. Theinvention is applicable likewise to any device in which the outputwindow, al-

though not circular (but rectangular), has a larger section than that ofthe tube output waveguide, thus necessitating a waveguide junction.

In a particular embodiment, an improved tube in accordance with'theinvention is a klystron operating at a frequency range extending from2900 to 3200 Mc/s and delivering peak powers of as much as 25 megawatts.For such a tube, the output window is made for example of glucine. It iscircular and thick its diameter being in the order of 68 mm and itsthickness 20 mm. The parasitic modes close to the operating band AF arein the T5 mode at frequency 2860 Mc/s and the TM mode at frequency 3220Mc/s they are thus outside said band AF. I

This window is arranged in a circular waveguide portion linked by twoconical junctions to the output and load waveguides which latter twocontain the two sets of metal obstacles in accordance with theinvention, as illustrated in FIG. 4 for example. Thanks to thisarrangement, the output system, throughout the band AP, has a standingwave ratio very close to unity and in any case of less than 1.1.Moreover, theshort length of the system constituted by the circularwave-guide system and its two junctions, its length is less than mm,avoid the appearance of any free-running parasitic resorrance.

' What I claim, is

l. A high frequency tube comprising, an exhausted enclosure, at leastone rectangular-output waveguide tightly connected to said enclosure, awaveguide portion of larger section than said output waveguide, aprogressive junction connecting said output waveguide and said waveguideportion, a dielectric output window arranged within said waveguideportion for maintaining the vacuum tightness of said tube enclosure,metallic obstacles, known as the first obstacles, arranged within saidoutput waveguide for ensuring to the system comprising said waveguideportion, said output window and said progressive junction, a resonancefrequency which is substantially equal to the central frequency of theoperating frequency band of said tube, and further metallic obstacles,known as the second obstacles, arranged within said output waveguide formatching the system comprising said waveguide portion, said outputwindow, said progressive junction and said first obstacles, throughoutthe operating frequency band of said tube.

2. A high frequency tube according to claim 1 comprising, a rectangularload waveguide of smaller section than said waveguide portion, a furtherprogressive junction connecting said load waveguide to said waveguideportion opposite to said output waveguide, further metallic obstacles,known as the third obstacles, said third obstacles being identical withsaid first obstacles and being disposed within said load waveguide in anarrangement which is symmetrical of that of said first obstacles withinsaid output waveguide with regards to said waveguide portion, andfurther metallic obstacles, known as the fourth obstacles, said fourthobstacles being identical with said second obstacles, and being disposedwithin said load waveguide in an arrangement which is symmetrical ofthat of said second obstacles within said output waveguide with regardsto said waveguide portion. a

3. A high-frequency tube according to claim 1, wherein said firstobstacles comprise two cylindrical metal bars located in a plane ofsection of said output waveguide and linking its wider faces to whichthey are perpendicular, said bars furthermore being symmetrically spacedfrom the axis of said waveguide and located in the neighborhood of thesmaller faces thereof, and wherein said second obstacles comprise twofurther cylindrical metal bars located in a further plane of section ofsaid output waveguide, said further plane being a little furtherawayfrom said output window than said plane of section of said firstobstacles, said further metal bars linking said output waveguide widerfaces to which they are perpendicular, said further metal barsfurthermore being symmetrically spaced from the axis of said waveguideand located closer to the smaller faces thereof than are the barsconstituting said first obstacles.

4. A high frequency tube according to claim 1, wherein said first andsecond obstacles are each constituted by two flat metal fins attached tothe smaller faces of said output waveguide in the same plane of sectionthereof, the plane of said first obstacles being closer to said'outputwindow than the plane of said second obstacles and said two planes beingin the vicinity of said progressive junction connecting said outputwaveguide and said waveguide portion.

5. A high-frequency tube according to claim I,

wherein said first and second obstacles are each constituted by two flatmetal fins attached to the larger faces of said output waveguide in thesame plane of section thereof, the plane of said first obstacles beingcloser to said output window than the plane of said second obstacles,and said two planes being slightly spaced away from said progressivejunction connecting said output waveguide and said waveguide portion.

6. A high-frequency tube according to claim 1, wherein the assembly ofsaid first and second obstacles is constituted by a resonant windowarranged within the output waveguide in the neighborhood of the junctionconnecting said output waveguide and said waveguide portion.

1. A high frequency tube comprising, an exhausted enclosure, at leastone rectangular output waveguide tightly connected to said enclosure, awaveguide portion of larger section than said output waveguide, aprogressive junction connecting said output waveguide and said waveguideportion, a dielectric output window arranged within said waveguideportion for maintaining the vacuum tightness of said tube enclosure,metallic obstacles, known as the first obstacles, arranged within saidoutput waveguide for ensuring to the system comprising said waveguideportion, said output window and said progressive junction, a resonancefrequency which is substantially equal to the central frequency of theoperating frequency band of said tube, and further metallic obstacles,known as the second obstacles, arranged within said output waveguide formatching the system comprising said waveguide portion, said outputwindow, said progressive junction and said first obstacles, throughoutthe operating frequency band of said tube.
 2. A high frequency tubeaccording to claim 1 comprising, a rectangular load waveguide of smallersection than said waveguide portion, a further progressive junctionconnecting said load waveguide to said waveguide portion opposite tosaid output waveguide, further metallic obstacles, known as the thirdobstacles, said third obstacles being identical with said firstobstacles and being disposed within said load waveguide in anarrangement which is symmetrical of that of said first obstacles withinsaid output waveguide with regards to said waveguide portion, andfurther metallic obstacles, known as the fourth obstacles, said fourthobstacles being identical with said second obstacles, and being disposedwithin said load waveguide in an arrangement which is symmetrical ofthat of said second obstacles within said output waveguide with regardsto said waveguide portion.
 3. A high-frequency tube according to claim1, wherein said first obstacles comprise two cylindrical metal barslocated in a plane of section of said output waveguide and linking itswider faces to which they are perpendicular, said bars furthermore beingsymmetrically spaced from the axis of said waveguide and located in theneighborhood of the smaller faces thereof, and wherein said secondobstacles comprise two further cylindrical metal bars located in afurther plane of section of said output waveguide, said further planebeing a little further away from said output window than said plane ofsection of said first obstacles, said further metal bars linking saidoutput waveguide wider faces to which they are perpendicular, saidfurther metal bars furthermore being symmetrically spaced from the axisof said waveguide and located closer to the smaller faces thereof thanare the bars constituting said first obstacles.
 4. A high frequency tubeaccording to claim 1, wherein said first and second obstacles are eachconstituted by two flat metal fins attached to the smaller faces of saidoutput waveguide in the same plane of section thereof, the plane of saidfirst obstacles being closer to said output window than the plane ofsaid second obstacles and said two planes being in the vicinity of saidprogressive junction connecting said output waveguide and said waveguideportion.
 5. A high-frequency tube according to claim 1, wherein saidfirst and second obstacles are each constituted by two flat metal finsattached to the larger faces of said output waveguide in the same planeof section thereof, the plane of said first obstacles being closer tosaid output window than the plane of said second obstacles, and said twoplanes being slightly spaced away from said progressive junctionconnecting said output waveguide and said waveguide portion.
 6. Ahigh-frequency tube according to claim 1, wherein the assembly of saidfirst and second obstacles is constituted by a resonant window arrangedwithin the output waveguide in the neighborhood of the junctionconnecting said output waveguide and said waveguide portion.